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Experimental Derivation of Nepheline Syenite and Phonolite Liquids By Earth and Planetary Science Letters 404 (2014) 319–331 Contents lists available at ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl Experimental derivation of nepheline syenite and phonolite liquids by partial melting of upper mantle peridotites ∗ Didier Laporte a,b,c, , Sarah Lambart d,1, Pierre Schiano a,b,c, Luisa Ottolini e a Clermont Université, Université Blaise Pascal, Laboratoire Magmas et Volcans, BP 10448, 63000 Clermont-Ferrand, France b CNRS, UMR 6524, LMV, 63038 Clermont-Ferrand, France c IRD, R 163, LMV, F-63038 Clermont-Ferrand, France d Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA e CNR-Istituto di Geoscienze e Georisorse (IGG), Unità di Pavia, Via A. Ferrata 1, 27100 Pavia, Italy a r t i c l e i n f o a b s t r a c t Article history: Piston-cylinder experiments were performed to characterize the composition of liquids formed at very Received 24 April 2014 low degrees of melting of two fertile lherzolite compositions with 430 ppm and 910 ppm K2O at 1 and Received in revised form 30 July 2014 1.3 GPa. We used the microdike technique (Laporte et al., 2004)to extract the liquid phase from the Accepted 4 August 2014 partially molten peridotite, allowing us to analyze liquid compositions at degrees of melting F down to Available online xxxx 0.9%. At 1.3 GPa, the liquid is in equilibrium with olivine + orthopyroxene + clinopyroxene + spinel in Editor: J. Brodholt all the experiments; at 1 GPa, plagioclase is present in addition to these four mineral phases up to about ◦ Keywords: 5% of melting (T ≈ 1240 C). Important variations of liquid compositions are observed with decreasing upper mantle temperature, including strong increases in SiO2, Na2O, K2O, and Al2O3 concentrations, and decreases lherzolite in MgO, FeO, and CaO concentrations. The most extreme liquid compositions are phonolites with 57% partial melting SiO2, 20–22% Al2O3, Na2O + K2O up to 14%, and concentrations of MgO, FeO, and CaO as low as 2–3%. phonolite Reversal experiments confirm that low-degree melts of a fertile lherzolite have phonolitic compositions, mantle metasomatism and pMELTS calculations show that the amount of phonolite liquid generated at 1.2 GPa increases from potassium 0.3% in a source with 100 ppm K2O to 3% in a source with 2000 ppm K2O. The enrichment in silica and alkalis with decreasing melt fraction is coupled with an increase of the degree of melt polymerization, 4+ which has important consequences for the partitioning of minor and trace elements. Thus Ti in our 4+ experiments and, by analogy with Ti , other highly charged cations, and rare earth elements become less incompatible near the peridotite solidus. Our study brings a strong support to the hypothesis that phonolitic lavas or their plutonic equivalents (nepheline syenites) may be produced directly by partial melting of upper mantle rock-types at moderate pressures (1–1.5 GPa), especially where large domains of the subcontinental lithospheric mantle has been enriched in potassium by metasomatism. The circulation of low-degree partial melts of peridotites into the upper mantle may be responsible for a special kind of metasomatism characterized by Si- and alkali- enrichment. When they are unable to escape by porous flow, low-degree melts will ultimately be trapped inside neighboring olivine grains and give rise to the silica- and alkali-rich glass inclusions found in peridotite xenoliths. © 2014 Elsevier B.V. All rights reserved. 1. Introduction were focused on partial melting of depleted mantle at mid-ocean ridges (where the mantle contains only 60 ppm K2O according Basaltic magmas at mid-ocean ridges, oceanic islands, intracon- to Workman and Hart, 2005) and thus used starting peridotites tinental rifts and subduction zones are mainly produced by partial either K2O-free (e.g., Falloon et al., 1999, 2008; Robinson et al., melting of upper mantle peridotites. Many experimental studies 1998; Presnall et al., 2002) or very poor in K2O (e.g., Baker and Stolper, 1994). Estimates of K2O in the primitive mantle (160 to 340 ppm; Taylor and McLennan, 1985; McDonough and Sun, 1995; * Corresponding author at: Laboratoire Magmas et Volcans, CNRS/UBP/IRD, 5 rue Arevalo et al., 2009, and references cited therein) are, however, up Kessler, 63038 Clermont-Ferrand Cedex, France. Tel.: +33 4 73 34 67 33. to 6 times higher than in the depleted MORB mantle, and there E-mail address: [email protected] (D. Laporte). 1 Now at: Lamont Doherty Earth Observatory, Columbia University, Palisades, NY are source regions in the upper mantle that may be even more 10964, USA. enriched (McKenzie, 1989). In particular, the lithospheric mantle http://dx.doi.org/10.1016/j.epsl.2014.08.002 0012-821X/© 2014 Elsevier B.V. All rights reserved. 320 D. Laporte et al. / Earth and Planetary Science Letters 404 (2014) 319–331 below continental and oceanic crust may be enriched in potassium in the microdike is not in close contact with peridotite minerals, it by repeated episodes of metasomatism: for instance, Menzies et al. is not modified by the growth of quench crystals at the end of (1987) describe metasomatized peridotite xenoliths in which K2O the experiment. In all our runs except MBK+7 and MBK15, we exceeds 1%. observed a few microdikes varying from tens to hundreds of mi- As K2O is highly incompatible with respect to most minerals crons in length and from a few microns to a few tens of microns during mantle melting, and as the incorporation of alkalis strongly in width, and yielding consistent glass compositions (Table 1). The affects the degree of polymerization of silicate liquids (Ryerson, length of the microdikes is short compared with the thickness of 1985; Hirschmann et al., 1998), even a moderate increase of K2O the graphite walls (0.7 to 0.9 mm) so that the partial melt is not in in the mantle source may have a dramatic effect on the major el- contact with the outer platinum container. This technique is per- ement composition and on the structure of partial melts, specially fectly suited to the study of low degrees of melting because the at low degrees of melting (≤5%; all percentages are wt.% except volume of the microdikes is very small in comparison with the to- when specified otherwise). These changes in melt structure may tal volume of partial melt in the sample. in turn have important consequences for the partitioning of minor A drawback of the experiments at low degrees of melting is that and trace elements (Gaetani, 2004; Huang et al., 2006). any trace of water in the capsule will partition preferentially into Here we present the results of melting experiments on two the partial melt, yielding significant amounts of dissolved water fertile, K2O-rich peridotites: composition MBK (410 ppm K2O) is even under nominally anhydrous conditions. Dissolved water con- within the range of the primitive mantle as estimated by Arevalo tents in some experimental glasses were measured using an ion et al. (2009): 340 ± 140 ppm K2O; composition MBK+ is signifi- microprobe with a beam diameter of 5μm (see Section 1 of the cantly more enriched (930 ppm K2O). Three series of experiments Supplementary material; Ottolini and Hawthorne, 2001; Ottolini et were performed: two with MBK at 1 and 1.3 GPa, and one with al., 1995, 2002; Scordari et al., 2010). The water contents are rela- MBK+ at 1.3 GPa. The pressures of 1 and 1.3 GPa were chosen tively low and decrease with increasing melt fraction F : from 1.1% ≈ = because the effect of alkalis on the structure of mantle melts is at our lowest degrees of melting (F 1%) to 0.1% at F 12.7%. more important below 1.5 GPa (Hirschmann et al., 1998). In ad- 3. Experimental results dition, this range of pressure straddles the transition from pla- gioclase + spinel lherzolite (at 1GPa) to spinel lherzolite (at 3.1. Experiments at 1.3 GPa 1.3 GPa), allowing to study the effect of plagioclase on low-degree melt compositions. We used the microdike technique (Laporte et Run information (P –T conditions, run products, modes, liquid al., 2004)to extract and analyze liquids at degrees of melting F compositions, etc.) is summarized in Table 1; compositions of solid down to 0.9%. Our low-degree melts are rich in Na2O + K2O (up phases are given in Supplementary Table S1. Liquid is in equi- to 14 %), SiO2 (up to 57%), Al2O3 (20–22%), poor in MgO, FeO, librium with olivine (ol) + orthopyroxene (opx) + clinopyroxene CaO (down to 2–3%), and classify as trachyandesites, tephriphono- (cpx) + spinel (spl) in all the experiments at 1.3 GPa (Fig. 1b). The lites and phonolites in the total alkali–silica (TAS) diagram (Le ◦ ◦ melt fraction F increases from 0.9% at 1180 C to 4.1% at 1270 C Bas et al., 1986). Calculations with the pMELTS software (Ghiorso ◦ ◦ in MBK, and from 1.6% at 1150 C to 5% at 1250 C in MBK+ (Ta- et al., 2002) confirm the experimental findings and document ble 1). Thus, at a given temperature, F is higher in the composition the effect of bulk K2O (up to 2000 ppm) on melt compositions with the larger K2O content. The absence of microdikes in sample and melt fractions. Lherzolites relatively rich in potassium (HK-66, ◦ MBK+7 (T = 1130 C) and its lower degree of textural equilibra- 0.07% K2O; PHN1611, 0.14% K2O) were studied by Kushiro and co- tion suggest that it could be subsolidus. In this case, a K-bearing workers (Takahashi and Kushiro, 1983; Hirose and Kushiro, 1993; solid phase, such as feldspar, must be present even if we did not Kushiro, 1996), but most of their experiments were at high degrees find it despite careful examination.
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